Speaker Impedence Levels

[trendkill1168]

What speaker impedence settings are ya'll using out there with the RM100 and one 4X12 cab? I have a Randall XXL cab and have the option of either running mine at 16 ohms mono, 8 ohms stereo, or 4 ohms mono. I am currently running mine at 4 ohms. I was under the impression that this would enable the head to run at full power.

 

[acez71]

I would like to know this too?

 

[muudrock]

Generally, the lower the impedance, the more output. But you are not going to notice a major difference in volume between 4 ohms or 16 in a modern class A/B tube amp. As long as your matching impedances on the amps output and the speakers your good. You can safely(although I don't recommend it for long periods of time) run your amp at a lower settings ie. amp@8ohms into speakers@16ohms without damaging anything. You are just lowering the total available output of the amp and making your output section work alot harder. I know alot of guys that do this to get a warmer sound at low volumes. However, NEVER run your amps output impedance set over that of the speakers ie: amp @ 16 ohms into Speakers @ 4 ohms. You will be replacing your output transformer and probably more very quickly. The output load (speakers) must always be equal to or higher than the output impedance of the tube head or things start blowing up.
That being said, Experiment. You may like the overall tone and feel better @ 16 ohms.

 

[trazan]

The output load (speakers) must always be equal to or higher than the output impedance of the tube head or things start blowing up.

This is the case with solid state amps, but with a tube amp it's actually the other way around. IOW, it's safer to run an 8ohm head into 4ohm than 16ohm load.

You'd probably be fine running an 8ohm head into either 4- or 16ohm, but I wouldn't try running a 16ohm head into 4ohms, or the other way around 8)

 

[muudrock]

This is the case with solid state amps, but with a tube amp it's actually the other way around. IOW, it's safer to run an 8ohm head into 4ohm than 16ohm load.

You'd probably be fine running an 8ohm head into either 4- or 16ohm, but I wouldn't try running a 16ohm head into 4ohms, or the other way around 8)

I disagree. You will damage a tube head by running into a load that is less than the amp is set for. The power section needs the "pushback" from the proper impedance load.

 

[Chad]

Not saying I recommend this, but I read an article fairly recently (wish I still had the link) and it said that most modern amps have beefy enough power supplies that you are okay to go one tap higher or lower. i.e. with a 8ohm setting on an amp, you could go with a 16ohm or 4 ohm load. But it's still definitely best to match speaker load with with the correct tap on the amp. The RM amps weigh a ton, so I'm guessing they have fairly beefy power supplies, but proceed at your own risk.

 

[trazan]

This is the case with solid state amps, but with a tube amp it's actually the other way around. IOW, it's safer to run an 8ohm head into 4ohm than 16ohm load.

You'd probably be fine running an 8ohm head into either 4- or 16ohm, but I wouldn't try running a 16ohm head into 4ohms, or the other way around 8)

I disagree. You will damage a tube head by running into a load that is less than the amp is set for. The power section needs the "pushback" from the proper impedance load.

You disagree? 8) Can higher powers please chime in on this (i.e. Bruce or similar..)? I'm no amp tech myself, so I'm only passing on info that have been given me. I've seen the same discussion several times before though, and to me it seemed that the most knowledgeable ones stated the opinion I posted above. Condensed :"A higher impedance load will reflect higher voltage spikes back to the output transformer and tubes, which can lead to damage to the OT or flashing at the tubes/sockets."

Most likely it's safe to go one step in either direction though.

 

[acez71]

The Rm100 head has 3 positions 4,8,16 as does the xl cabinet,what is recommended to use?

 

[Chad]

The Rm100 head has 3 positions 4,8,16 as does the xl cabinet,what is recommended to use?

The 8 ohm on the XL cabinet is for stereo use only, isn't it? If so, you don't want to use that unless you only want to use one half of the cabinet.

I'd say set the amp to 4 ohms, then plug into the 4 ohm jack on the cabinet and listen. Then set the amp to 16 ohms and plug into the 16 ohm jack on the cabinet and listen again. Then pick the one that you like the best. There isn't really a wrong or right in that situation.

 

[kewlpack]

Jumping in here -

I have an RM50 Head and two 1x12 cabinets. Both are 8ohm. I was going to use the MAIN and EXTENSION outputs on the Head to feed a 1x12 each. What impedence should I set the Head to? Currently, I have it set to 4ohm, but one cabinet is WAY louder than the other.

The cabinets are:

Mesa Boogie Oversized 1x12 C90 (Black Shadow 90 watt)
Crate GT112SL 1x12 (Celestion G12T-100 100 watt)

I actually prefer the tone from the low end Crate 1x12 - much more articulate and open sounding. The Crate is also much louder than the Mesa (go figure).

Can someone help me out please.

 

[Chad]

Are you sure they're both 8 ohm cabinets? I'd look at both of the speakers and see if they have the ohm rating listed on them. Or hook an ohm meter up to the terminals and see if they are in range.

If they are indeed both 8 ohm cabinets, then, yes, you'd want to go with the 4 ohm setting on the amp because the extension jack parallels with the main speaker output jack.

By the way, I'd try to find the specs for the speakers as well. If one has higher sensitivity (more efficient) that could make a speaker louder.

 

[kewlpack]

Yepper - they're both rated at 8ohm.

The cabinet designs are markedly different, and the speakers themselves are different. I suspect it's just a design variance that makes the Crate cabinet much clearer... hmmm. :?

 

[Chad]

I've got a feeling the Celestion speaker might have a higher sensitivity rating than the Black Shadow.